This invention relates to methods of manufacturing semiconductor devices and, more particularly, to a method of manufacturing a semiconductor device in which a high breakdown voltage semiconductor element and a low breakdown voltage semiconductor element are formed on the same semiconductor substrate.
Semiconductor devices, in which a high breakdown voltage bipolar transistor and a low breakdown voltage bipolar transistor are respectively formed within a high breakdown voltage element region and a low breakdown voltage element region, these regions being defined in a single semiconductor substrate, are in practical use. In the semiconductor device of this structure, an isolation region is provided between the high breakdown voltage and low breakdown voltage bipolar transistors.
In the prior-art method of manufacturing this semi-conductor device, the isolation region is formed by diffusing an impurity from the surface of the eventual device in the last or practically last step of the manufacturing process. This means that the diffusion at this time has to be made considerably deep. Therefore, it is inevitable that the isolation region spreads laterally a considerable distance, i.e., along the surface of the device. Namely, this region occupies a considerable area, which is undesirable from the standpoint of the integration density.
In another aspect, a high impurity concentration buried layer is provided within the low breakdown voltage element region. With the prior-art device the distance between this buried layer and the collector of the low breakdown voltage bipolar transistor is comparatively large. Therefore, the collector series resistance between that collector and the buried layer is considerably high. This means that the saturation characteristic, i.e., the collector saturation voltage, of the low breakdown voltage bipolar transistor is considerably high.